(a) Technical Field
The present invention relates to a driving apparatus and method for a modular multi-level converter. More particularly, it relates to a driving apparatus and method for controlling submodules by an upper valve or a lower valve of a valve branch taking charge of each phase.
(b) Background Art
When energy needs to be transported from an offshore wind farm to a main land using a submarine cable of about 100 kilometers or more, it is known that the High Voltage Direct-Current (HVDC) transmission is much more economical than the High Voltage Alternating-Current (HVAC) transmission in terms of energy transportation cost.
The HVDC transmission is known as an appropriate method particularly for international power transaction (energy trade) in which countries may use different frequencies and voltages. Also, as it is demonstrated that the High Voltage Direct-Current (HVDC) transmission is much more economical than the High Voltage Alternating-Current (HVAC) transmission in terms of energy transportation cost even when an energy bottleneck phenomenon occurs due to the extensive energy consumption in downtowns that are densely populated areas, new attention is being given to the HVDC transmission.
Particularly, when solar energy and wind energy abundantly distributed in the African continent can be developed and transported to the European continent, the new and renewable energy share can be significantly increased in Europe. Thus, this technology is most developed in Europe. Also, the new and renewable energy market is being rapidly growing in China that needs to transport large-capacity hydroelectric power stations to large cities away therefrom by about 1,000 kilometers and can produce energy from deserts.
When the HVDC transmission systems are classified according to the type of the converter, the HVDC transmission systems may be classified into an HVDC transmission system having a current-type converter and an HVDC transmission system having a voltage-type converter. The present disclosure relates to a voltage-type converter, and more particularly, to a modular multi-level converter among the voltage-type converters.
In the modular multi-level converter, a unit submodule is manufactured using an Insulated Gate Bipolar Transistor (IGBT) with a low voltage specification, and the submodules are stacked in series to form a stack structure with a withstanding voltage ability with respect to a high voltage of hundreds of KVs. Also, the modular multi-level converter is allowed to have a variety of voltage levels according to the number of submodules stacked in series.
In addition, the modular multi-level converter can perform independent control of active power and reactive power which cannot be implemented in a HVDC transmission system having a current-type converter, and need not together supply reactive power corresponding to 50% of active power in order to transmit active power. Also, each of converters located at the both ends of a high DC voltage can be stably controlled without information on the counter converter, and the transportation direction of active power can be simply controlled by changing only the current direction without a process of re-determining the magnitude of voltages at the both ends.
However, due to its structure, the modular multi-level converter for the HVDC transmission has limitations that the current-type converter does not have.
In other words, since the capacity voltage in the submodule is not uniform and a resultant voltage of an upper valve voltage and a lower valve voltage is not the same as a DC link voltage, there are limitations in that a circulating current component flowing in the multi-level converter exists. Also, a harmonic may be induced in a high voltage DC-grid, or a harmonic is contained in active power of an Alternating Current (AC) grid.
In order to overcome these limitations, various methods have been proposed. However, when an accident such as one-phase earthed occurs, the AC-grid voltage becomes an unbalance voltage state. In this condition, the circulating current is not suppressed, or a harmonic is induced in the DC-grid. Also, a harmonic is contained in active power of the AC-grid, showing that the control characteristics are still not strong.
Also, a typical control method for the HVDC transmission system with a modular multi-level converter requires an upper controller to perform a large amount of operation, and is difficult to implement in a Valve Controller (VC). Accordingly, the typical control method for the HVDC transmission system is not suitable for improvement of the operation speed. Specifically, since a current reference value derived from an active and reactive power controller and a current reference value derived from a circulating current suppression controller use a phase current, or use an expression of the phase current transformed into a d-q coordinate plane, the typical control method is suitable to implement the controller by phase unit but is difficult to implement in each valve control, making it difficult to improve the operation speed.
(c) Prior Art